Clearance control



April 10, 1951 GARR 2,548,599

CLEARANCE CONTROL Filed NOV. 2, 1948 2 Sheets-Sheet 1 Figl.

Inventor: Donald. ELGar- T,

His Attor-n ey.

April 10, 1951 Filed Nov. 2, 1948 D. E. GARR CLEARANCE CONTROL Fig. 5.

2 Sheets-Sheet 2 Inventor: Donald BGaPh,

His Attorney.

Patented Apr. 10, 1 951 CLEARANCE CONTROL Donald E. Garr, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application November 2, 1948, Serial No. 57,925

1 Claim. 1

This invention relates to a clearance control, and in particular to apparatus for controlling the clearance between two refining plates of a rotary disk pulp refiner.

The principal object of this invention is to provide an improved automatic clearance control of the type described. Other objects and advantages will appear as the description proceeds.

The features of the invention which are believed to be novel and patentable are pointed out in the claim forming a part of this specification. For a better understanding of the invention, reference is made in the following description to the accompanying drawings in Which Fig. 1 is a schematic diagram, partly in cross section, showing attachment of the clearance control to a rotary disk pulp refiner; Fig. 2 is a cross sectional view drawn to a larger scale showing the relation between the refining disks and a reactor used in the control system; Fig. 3 is a view perpendicular to the plane of Fig. 2; Fig. 4 is a circuit diagram of one form of the control system; and Fig. 5 is a circuit diagram of another form of the control system. Where the same part appears in more than one figure of the drawing, it is always represented by the same reference numeral.

Refer now to Fig. l, which shows the attachment of control system elements to a rotary disk pulp refiner. The pulp refiner includes a steel base I, feed end shell 2, feed spout 3, stationary refining plate 4, runner head 5, rotating refining plate 6, shaft I, feed end bearing 8, packing rings 9, electric motor I0, thrust bearing II, thrust bearing screw I2, tail end pedestal l3, and tension bar I4.

Water and pulp to be refined are fed in through feed spout 3. The pulp passes between refining plates 4 and 6, and the refined pulp escapes at the bottom of feed end shell 2. The fineness of the refined pulp is determined by the clearance between the refining plates 4 and 6. This clearance is regulated by thrust bearing I I under the control of thrust bearing screw I2.

According to a prior art method of operating such refiners, the clearance is adjusted manually as follows: Without any material passing through the refiner, the operator adjusts the thrust bearing until a noise is heard, which indicates that the plates are touching. Micrometer dials, not shown, are then set, and the bearing is retracted a distance of a few thousandths of an inch which the operator feels to be correct for the particular pulp to be refined. After the adjustment is made, any change in motor temperature, or

any change in refiner load, will cause the axial dimension of the shaft and other parts to change. Therefore, adjustments must continually be made, which if done manually necessitates considerable delay with a consequent reduction in refiner output, in addition to requiring frequent attention by a skilled operator. Refiners have also been used with other types of control: for example, some have the clearance adjusted by a function of load current of the main driving motor. However, such systems do not control the clearance with the desired degree of accuracy.

In one form of my improved apparatus to control the clearance between plates 4 and 6 automatically, a reversible motor I5 is connected through suitable gearing to turn thrust bearing screw [2; and a first reactor having a U-shaped magnetic core I6 is attached adjacent to plate 4 as shown in Figs. 1, 2, and 3. Plates 4 and 6 are usually made of hard steel, which is a magnetic material. A hole is cut in plate 4 and filled with an insert [1 of relatively soft, non-magnetic and preferably non-conducting material such as wear-resistant plastics. In tests which have been conducted, it has been found that wear of such a soft insert proceeds at substantially the same rate as wear of the harder refining plates. U-shaped core I6 is positioned with its open end adjacent to insert IT, as shown. Legs I8 of magnetic material, such as soft iron, extend from the ends of reactor core I6 through the insert, so that the magnetic flux path of the reactor includes core I6, legs I8, the space between refining plates, and a portion of plate 6. The reluctance of this magnetic path is determined almost entirely by the clearance between plates 4 and 6, since all other parts of the path are of magnetic material having relatively small values of reluctance. Windings I9 connected in series are provided about core IS in the conventional manner.

Refer now to Fig. 4, which is a circuit diagram of the control system. A second reactor has a U-shaped core 20 and series-connected windings 2 l. Preferably, the first and second reactors are substantially identical. A magnetic member 22 is adjustably spaced from the open end of core 20. The magnetic path of the second reactor thus includes core 20, member 22, and the space therebetween; and its reluctance is almost entirely determined by the space. Thus it is evident that if clearance between plates 4 and 6 is equal to the space between core 20 and member 22, both reactors will have magnetic paths of substantially the same reluctance, and accordtween the two air gaps.

ingly the alternating current impedance of the two reactors will be substantially the same. In operation of the system, the space between core 20 and member 22 is adjusted to the desired clearance between refiner plates by a micrometer screw 23, which may be provided with a suitable calibrated dial, not shown.

The two reactors are connected in respective legs of an alternating-current bridge circuit. The other two legs of the bridge circuit comprise capacitors 2d and 25 and resistors 26 and 21. The bridge is energized by voltage from onehalf the secondary of a transformer 28, the primary of which is supplied with alternating current from a suitable source through connections 2! Capacitors 24 and 25 resonate with the reactors, and thus accentuate vany difference be- This gives the bridge circuit greater sensitivity. It is evident that other bridge circuits may be used to give equivalent results, and that other impedances may be substituted for the second reactor. However, the form shown is preferred because of its simplicity and convenience of operation.

A transformer 33 has a primary connected across the output of the A.-C. bridge, and a divided secondary connected in the grid circuit of a dual triode vacuum tube 35, as shown. Since the two grids are connected to the opposite sides of the transformer secondary, the grid voltages are in phase opposition. The plates of tube 3| receiv alternating voltage from the secondary of transformer 28. Relays 32 and 33 are respectively connected in series with the vacuum tube plates. Capacitors 3-: and 35 are connected in parallel with the two relays in a conventional manner to smooth the currents through the relays and thus improve their operation. Resistors 35, 31, 38, 39, and it perform conventional cathode bias and grid leak duties in the vacuum tub circuit.

Vacuum tube 3| and its associated circuit, together with relays 32 and 33, functions a a phasesensitive relay which responds to unbalance of the alternating current bridge circuit. When the bridge circuit is balanced, cathode bias resistor 35 is adjusted so that each half of vacuum tube 3| conducts a value of current just below that required to operate relays 32 and 33. Thus both relay are de-energized at balance, but any increase in current through either half of tube 3| will energize the corresponding relay. Other forms of phase-sensitive relays are known in the art which may be substituted for the relay shown without departing from the principle of the invention.

When relay 32 is energized, it closes normally open contacts 4|, thus energizing relay 52. This close normally open contacts 43, which causes motor to rotate to the right and turn the thrust bearing screw to increase the clearance between plates 1 and 6. When relay 33 is energized, normally open contacts t l are closed. This energizes relay 35 which closes normally open contacts as and causes motor E5 to rotate to the left and reduce the clearance between plates 5 and 6. Normally closed contacts ll and t8 prevent simultaneous energization of relays 22 and '35, which would short-circuit the alternating current supply.

In the operation of the system, when for any reason the clearance between plates i and 6 becomes less than the standard clearance established by reactor core 29 and member 22, the alternating current bridge circuit is unbalanced in such a direction that an alternating voltage is 4 applied to the left-hand grid of tube 3| which is in phase with the voltage applied to the plates. Thus the left-hand grid of tube 3| is driven positive at the same time that positive voltage is applied to the plates, and consequently the conduction of current through the left-hand side of tube 3| is increased. This energizes relay 32 and causes motor I5 to rotate to the right, as has been explained, which increases the clearance between plates 4 and 8 until the bridge circuit is returned to balance. The alternating voltage applied to the right-hand grid of tube 3| is in phase opposition to the voltage applied to the plates, so that the right-hand grid is driven negative when the plates are positive, and conduction through the right-hand side of the tube is reduced. Thus relay 33 remains unenergized. Conversely, if for any reason the clearance between plates i and 6 becomes greater than the standard, the voltage applied to the right-hand grid of tube 3| is in phase with the voltage applied to the plates, relay 33 is energized and motor l5 rotates to the left to decrease the clearance between plates 4 and 6 until the bridge circuit is returned to balance.

Push buttons 59 and 58 are provided as a means for manually operating motor E5 to increase or decrease the clearance between refiner plates independent of the operation of the automatic control system, when it is desired to do so. Switch 5? may be opened to shut off the automatic control without afiecting the push button control. Normally closed limit switches 52 and 53 may be provided to prevent over-travel of the thrust bearing in either direction. 1

Refer now to Fig. 5, which is-a circuit diagram of another form of the invention. In this modification two drive motors Ida and i519 are respectively connected through suitable gearing to the thrust bearing screw of the refiner. These two motors may be three-phase induction motors, as shown. Motor |5a turns the thrust bearing screw in a direction to close the gap between the refiner plates. This motor is connected to the thrust bearing screw through gears having a large speed-reduction ratio, so that the gap between the refiner plates is closed relatively slowly, which makes for more accurate positioning of the plates. Motor 55b is geared to retract the feed screw, and

' thus separate the refiner plates, at a very rapid speed. This prevents the refiner plates from coming together when the water supply and pulp feed is lost.

The two reactor assemblies in the apparatus of Fig. 5 may be identical with those in the apparatus of Fig. 4. The reactors plus resistors 54 and 55 comprise a bridge circuit across which an error voltageappears responsive to any deviation of the refiner plates from their pre-selected clearance. This error voltage is applied to the primaries of transformers 56 and 57a, which are connected in such a way that voltages of opposite polarities are applied to the left-hand and right-hand control grids respectively of vacuum tube 58. Depending upon the direction of unbalance of the bridge circuit, the voltage applied to one of the control grids will be in phase with the plate voltage, while the voltage applied to the other control grid is in phase opposition to the plate voltage. The in-phase voltage is amplified and applied to the corresponding control grid of vacuum tube 5%}, where it is again in phase with the plate to cathode voltage, and causes its section of tube 59 to conduct heavily. The outof-phase voltage is suppressed, and has a negligible efiect upon the conduction of its side of vacuum tube 59. Tubes 58 and 59 with their associated circuits thus constitute a phase-sensitive relay.

Relays 6G and BI are connected in respective plate circuits of the two sections of vacuum tube 59. These relays are normally de-energized and in the position shown in the drawing. When an error signal causes one section of vacuum tube 59 to conduct more heavily, the associated relay is -irergized. When relay 60 is in its energized position, connections are completed to energize relay 62, which operates motor la to decrease the gap between refiner plates. When relay 6| is energized connections are completed to energize relay which operates motor l5b to retract the thrust bearing screw and increase the gap between refiner plates. A limit switch 64 is provided to prevent over-travel on retraction of the thrust bearing screw. Push buttons 65 and 66 are connected in the circuit to energize relays 62 and 63 respectively for manual operation of motors I5a and 1517.

When no motion is required, and consequently both relay 62 and relay 63 are de-energized, a small amount of direct current from rectifier 6! is passed through the motor windings, thereby causing a braking action which tends to halt any motion of the thrust bearing screw. This makes it practical to have a much more sensitive control without over-shoot or instability troubles. The amount of D.-C. fed to the motor windings is ,controlled by adjustable resistor 68. Adjustable resistor 89 is provided to control the ratio of direct currents fed to motors 15a and I5!) respectively,

Capacitor I0 and resistor H provide a time delay which prevents immediate energization of relay 60 after operation of relay Bl. This arrangement reduces hunting which might be caused by rapid alternate operation of the two relays.

Having described the principle of this inven tion and the best mode in which I have contemplated applying that principle, I wish it to be understood that the apparatus described is illustrative only, and that other means can be employed without departin from the true scope of the invention defined by the following claim.

What I claim as new and desire to secure by Letters Patent of the United States is:

Apparatus for controlling the clearance between two refining plates of a rotary disk pulp refiner having a thrust bearing screw for adjusting such clearance, comprising a non-magnetic insert extending through one of the refining plates, an alternating-current bridge circuit including first and second reactors each having U.-shaped cores, the first reactor core being positioned with its open end adjacent to said insert on the side opposite the other refiner plate, magnetic legs extending from the ends of the first reactor core through said insert, a magnetic member adjustably spaced from the open end of the second reactor core, a phase-sensitive relay connected to respond to unbalance of the bridge circuit, and a motor controlled by said relay and connected to turn the thrust bearing screw.

DONALD E. GARR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,984,869 Farley Dec. 18, 1934 41,311 Meeker Dec. 27, 1938 2,269,152 Hathaway Jan. 6, 1942 2,293,502 Hermann Aug. 18, 1942 2,320,881 Newton June 1, 1943 2,519,367 Gunn Aug. 2 1950 

